Logging tools for measuring the formation density in open holes are well known. Currently-available "open-hole" density logging tools normally include a gamma ray source, typically cesium-137, and two detectors--a near detector and a far detector. Open-hole density tools have two detectors to compensate for mudcake (i.e. a layer of solid material consolidated from drilling fluid that normally lines an open borehole) and standoff (i.e. the distance between the tool and the side of the borehole). Gamma rays are continuously emitted from the source and propagate out through the mudcake and into the formation. The electron density of the formation is calculated based on the count rate or intensity of the gamma rays that are received at the detectors after passing through the mudcake and formation.
In addition to employing radioactive sources for gamma rays, it is anticipated that density tools will employ accelerators, or more specifically electron accelerators, as gamma ray sources even though such accelerators are not currently available. In tools employing accelerators, the detectors will not necessarily detect and/or calculate gamma ray count rates because most accelerator designs generate relatively huge amounts of gamma rays for brief, intermittent periods of time resulting in large quantities of gamma rays engaging the detectors within a relatively short period of time. Therefore, it may be more feasible to detect and measure the intensities of the gamma rays rather than the count rate or the rate at which gamma rays engage the detectors. The detectors would generate voltage signals proportional to the intensities of the detected gamma rays as opposed to count rate signals.
In general, a layer of mudcake exists along the outer periphery of the borehole. The intensity signals from the near and far detectors are combined to provide a measure of the formation density that is essentially independent of this mudcake if the thickness of the mudcake is within a limited range. This is possible when the spacings and collimations chosen for the two detectors result in substantially different sensitivities to the mudcake and formation. The thickness of the mudcake that can be compensated for is limited by the spacing between the near detector and the gamma ray source and by the collimation of the near detector. A longer spacing between the gamma ray source and the near detector enables more mudcake to be penetrated by the gamma rays and therefore enables the tool to compensate for thicker mudcake. However, greater spacings between the gamma ray source and the near detector reduce the contrast between the near and far detector sensitivities to mudcake and the formation, thereby reducing the quality of the density measurement.
The two-detector density logging tools discussed above are limited in their use in that they are unable to provide an accurate formation density when used in a cased hole. Specifically, while the currently-available two detector density tools can compensate for mudcake, they are unable to compensate for the casing and cement and are therefore unable to accurately calculate the formation density in a cased hole.
The gamma ray detectors consist of sodium iodide crystals connected to photomultiplier tubes. These devices are commercially available and well known in the art. When gamma rays interact in the sodium iodide crystal, they produce light which is converted by the photomultiplier tube into an electronic signal. These signals are amplified to produce voltage pulses that are proportional to the energy deposited in the crystal. These voltage pulses may be counted to determine the gross number of gamma rays detected per second or they may be sorted by amplitude to determine the number of gamma rays detected per second in various energy ranges. The later technique is common in open-hole logging, as described in the paper "A New Approach to Determining Compensated Density and P.sub.e Values with a Spectra-Density Tool", presented by G. L. Moake at the Annual SPWLA Logging Symposium in Midland, Jun. 16-19, 1991.
Typical two detector density tools are decentralized within the borehole by a bowspring or caliper device so that the detectors and the source are pushed up against the side of the borehole. The decentralization of the tool against the side of the borehole substantially precludes the gamma rays from propagating upward through the drilling fluid and therefore the effect of the drilling fluid on the density measurement is minimized.
However, as noted above, the two detector density tools cannot adequately function in cased holes because they are unable to compensate for the casing and the cement. In short, too many variables are present in order to accurately calculate the formation density. Specifically, in order to probe the formation, the gamma rays must exit the tool, pass through the casing and cement and scatter in the formation before passing back through the cement and the casing to finally reenter the tool to be detected. Thus, instead of just a mudcake correction as in the case of open holes, a cased hole density tool must be able to correct or compensate for the cement and casing which are both more formidable a correction than a layer of mudcake because of the relatively high densities of metal casing and thick amounts of cement (i.e. gamma rays pass through a layer of mudcake more easily than typical casing and a layer of cement before reaching the near detector).
Occasionally, open hole density tools are used to run cased hole density logs. The information provided by open hole density tools in cased holes is accurate in only a limited number of cases, primarily when the cement thickness is quite small or no cement is present. This can occur on the low side of deviated wells where the casing may very well be resting against bare formation or only a small amount of cement will be disposed behind a casing. Typically, open hole density tools employ a gamma source in the form of contained cesium-137 which produces 0.662 MeV gamma rays. The amount of cement that can be corrected for is limited by the near or "first" detector. Typically, these near detectors can correct for only about 0.75 inches of mudcake in an open hole and therefore it is easy to see why these tools are ineffective in compensating for steel casing, which is normally 0.36 to 0.50 inches thick in combination with a layer of cement disposed behind the casing. In any event, even dangerously thin casing is much denser, and therefore more effective at blocking the passage of gamma rays, than mudcake.
As the gamma rays pass through the casing, cement and formation, they are scattered and absorbed by these materials. The primary scattering interaction is called Compton scattering and is caused by electrons in the casing, cement and formation. Compton scattering tends to reduce the energy of the gamma rays that are scattered until their energy is so low that the gamma rays are absorbed by electrons through an interaction called photoelectric absorption. Thus, if the density of electrons in the casing, cement and formation is high, fewer electrons will make it back through the formation, cement and casing to be detected at the tool. In short, the number of detected gamma rays varies inversely to the density of electrons in the various materials through which the gamma rays must pass. Since the density of electrons is directly related to the density of the material being measured, the number of detected gamma rays will also depend strongly on the density of the casing, the density of the cement and the density of the formation.
Two other physical processes that dominate the scattering and absorption of gamma rays are known as coherent scattering and photoelectric absorption, both of which are highly dependent upon the composition of the material. Those skilled in the art commonly represent the strength or effect of the coherent scattering and photoelectric absorption interactions relative to the strength of Compton scattering with the variable P.sub.e. For purposes of this invention, P.sub.e will therefore be defined as the strength or effect of the coherent scattering and photoelectric absorption interactions relative to the strength of the Compton scattering. Thus, the amount of gamma rays detected at the tool will also depend on the P.sub.e of the casing, the P.sub.e of the cement and the P.sub.e of the formation. Of course, the detected gamma rays will also depend on the thickness of the casing and the thickness of the cement.
As stated above, the performance of the tool will also depend slightly upon the density of the borehole fluid. However, density tools are commonly decentralized in the borehole, meaning that the tool, including the source and detectors is pushed up against the side of the borehole. In that manner, there is very little borehole fluid through which the gamma rays must pass in order to be detected at the tool. Further, since the borehole fluid properties are usually known and in any event can be tested at the surface, small correction factors based on the knowledge of the borehole fluid properties can be easily utilized.
Accordingly, there are eight variables that affect the response of the tool: the density of the casing (.rho..sub.s); the thickness of the casing (t.sub.s); the P.sub.e of the casing; the density of the cement (.rho..sub.c); the P.sub.e of the cement; the thickness of the cement (t.sub.c); the formation density (.rho..sub.f); and the P.sub.e of the formation. However, all but four independent variables can be accounted for prior to measurement.
First, casings tend to be made of the same material, namely carbon steel. Therefore, casings tend to have about the same density (.rho..sub.s) and P.sub.e. Thus the density (.rho..sub.s) and P.sub.e of the casing are known and the thickness (t.sub.s) of the casing is unknown.
Second, the P.sub.e of most cements is a negligible value because the low energy gamma rays that are sensitive to the P.sub.e of the cement are blocked by photoelectric absorption that occurs within the casing. Accordingly, only the density (.rho..sub.c) and thickness (t.sub.c) of the cement are important and unknown.
Further, the P.sub.e of the formation can also be neglected because P.sub.e, is a much lower value for high energy gamma rays than for low energy gamma rays. In other words, the effect of coherent scattering and photoelectric absorption is very low for high energy gamma rays; in contrast, the effect of coherent scattering and photoelectric absorption is relatively significant for low energy gamma rays. However, any gamma rays that become low energy gamma rays in the formation are unlikely to penetrate the cement and casing before being detected by the tool. Thus, the measured intensity at the tool will not depend significantly on the P.sub.e of the formation. Thus, the density (.rho..sub.f) of the formation remains unknown.
Therefore, in light of the above, there are only four independent variables that significantly affect the measured intensities: the density of the formation (.rho..sub.f); the thickness of the cement (t.sub.c); the density of the cement (.rho..sub.c); and the thickness of the casing (t.sub.s). Because there are four unknown variables, four independent intensities must be measured in order to determine the formation density in a cased hole. Further, assuming a casing density (.rho..sub.s), one can also calculate the casing thickness (t.sub.s), cement thickness (t.sub.c) and cement density (.rho..sub.c).
Accordingly, it would be highly desirable to provide a density logging tool with at least four detectors that would be capable of measuring the formation density in a cased hole as well as cement thickness, cement density and casing thickness. An effective cased hole density logging tool in combination with already existing cased hole neutron logging tools would provide an effective means for finding additional gas reserves behind existing cased holes. Further, an effective cased hole density tool would also provide important information regarding formation evaluation behind the casing. It is also contemplated that an improved cased hole density logging tool could also prove useful in open holes.
Still further, it is anticipated that an improved four-detector cased hole density logging tool will also improve the porosity measurement generated by neutron tools used in combination with a cased hole density logging tool. Specifically, in cased holes, the casing thickness, cement thickness and cement density are often estimated and assumed to be constant. However, neutron tools are very sensitive to casing thickness, cement thickness and cement density. Hence, assuming these variables to be constant and assigning a value to these variables can often result in significant errors in the neutron-porosity measurement. Therefore, when using an improved cased hole density tool in combination with a neutron-porosity tool, an operator can measure casing thickness, cement thickness and cement density and use these measured values to improve the neutron-porosity measurement.
Typical neutron-porosity tools consist of a neutron source and two neutron detectors. One detector is positioned closer to the source than the other. Each detector produces one count rate, which is indicative of the number of neutrons per second that pass through it. The tool is designed so that the bulk of the neutrons that reach the detectors have passed through the formation, where some of them are scattered and absorbed. The number of neutrons that ultimately reach the detectors depends strongly on the amount of hydrogen in the formation. When the formation contains water or oil in its pore spaces, the hydrogen content is strongly dependent on the porosity of the formation. Thus, an algorithm is used to convert the measured count rates to an estimate of formation porosity.
This algorithm usually requires the borehole diameter as an input. If not available, a constant value is assumed. The final answer is very sensitive to the diameter value that is used, so it is important to have an accurate estimate of it. The tool response is also very sensitive to the distance from the tool to the formation wall. During logging, the tool is pushed as close as possible to the borehole wall, since significant errors may occur if the tool is not flush against the wall. However, corrections can be made to the porosity estimate if the distance between the tool and wall is known.
In a cased hole, the neutron tool is pushed against the casing wall, but it cannot be pushed against the formation wall because of the presence of the casing and cement. Since the tools that measure borehole diameters in open holes do not work in cased holes, an estimate of the borehole diameter must be made. Since boreholes are generally rugose and have a varying diameter, the value assigned to the diameter will often be sufficiently incorrect to cause significant errors. This problem could be alleviated if a dynamically-varying measure of the borehole diameter were available. Since the tool is not pushed against the wall, its measurements will also be affected by the casing and cement. Since there was previously no way to measure the casing thickness and cement thickness, an estimate of these values was made. However, the amount of cement between the tool and formation will often be sufficiently incorrect to cause significant errors. This problem could be alleviated if a dynamically-varying measure of the cement thickness were available. Also, if the casing thickness is not accurately known, either because of erosion or missing records, errors will occur. Again, a dynamically-varying measure of the casing thickness would solve that problem.
An improved tool for measuring formation density in cased or open holes is provided in the form of a housing that accommodates a gamma ray source and at least four detectors. The gamma ray source is spaced axially from the first, second, third and fourth detectors. The first or near detector is axially spaced from the gamma ray source by a distance defined as a first spacing. The first spacing and collimation for the first detector are designed so that the gamma rays detected at the first detector are those gamma rays that are scattered primarily by the casing. In short, the first detector primarily detects or preferentially detects gamma rays scattered through the casing.
A second or near-middle detector is spaced axially farther away from gamma ray source than the near detector. The near-middle detector is spaced from the gamma ray source by a distance defined as a second spacing. The second spacing and collimation for the near-middle detector are designed so that the gamma rays detected at the near-middle detector will be those that are primarily scattered by the casing and shallow portions of the cement or the portion of cement adjacent to the casing.
A third or far-middle detector is spaced actually farther away from the gamma ray source than the near and near-middle detectors. The far-middle detector is spaced from the gamma ray source by a distance defined as a third spacing. The third spacing and collimation for the far-middle detector are designed so that gamma rays detected by the far-middle detector will be those that are primarily scattered by the casing, cement and formation. The far-middle detector will be more sensitive to those gamma rays scattered by the deep portion of the cement or the portion of the cement adjacent to the formation.
Finally, a fourth or far detector is spaced axially farther away from the gamma ray source than the near, near-middle and far-middle detectors by a distance defined as a fourth spacing. The fourth spacing and collimation defined by the far detector are designed so that the gamma rays detected at the far detector are those primarily scattered from the casing, cement and formation.
Preferably, the detectors are shielded by a high density material that prevents detection of gamma rays that are simply travelling up through the tool. A pathway or void in the shielding is provided in the form of a collimation channel which extends from the detector through the tool and terminates at the outside surface of the tool. The collimation channels are specifically designed for the purpose of each detector. Specifically, the near or first detector will have a collimation that is directed at a steep angle with respect to the casing so that the first detector will detect gamma rays that are scattered through all of the casing. The second or near-middle detector will have a collimation that is directed at a shallower angle with respect to the casing because the near-middle detector is intended to detect gamma rays scattered through the shallow portion of the cement as well as the casing. The third or far-middle detector will have a collimation directed at an angle so that the far-middle detector will detect gamma rays scattered through the casing, the deeper portion of the cement as well as a shallow portion of the formation. Finally, the fourth or far detector will have a wide collimation channel which is directed substantially perpendicular to the casing due to the distance of the far detector from the source. Because gamma rays detected at the far detector must pass through the casing, cement, and deep into the formation before passing back through the cement and casing, the statistical probability of this event happening is smaller than for the first and second detectors and therefore a wider collimation channel is required for the far detector.
It is also anticipated that a high energy gamma ray source such as cobalt 60 which provides 1.173 and 1.333 MeV gamma rays will be utilized. Other gamma ray sources, such as cesium-137 which provides only 0.662 MeV gamma rays or accelerator sources such as an electron accelerator could also be used. However, the spacing between the third or fourth detector and the gamma ray source will be longer than the conventional spacing between the gamma ray source and far detector in a open hole tool and therefore a source which emits higher energy gamma rays (i.e., 1.173 and 1.333 MeV) will provide a higher intensity at the far detector.
The present invention also lends itself to an improved method of measuring formation density behind casing as well as cement thickness and casing thickness. Specifically, the casing, cement and formation are irradiated with gamma rays. Gamma rays are detected at a first detector spaced a first axial distance from the gamma ray source. A first intensity is calculated for the first detector. Gamma rays are also detected at a second detector disposed a second axial distance from the gamma ray source. The second detector being disposed farther from the source than the first detector. A second intensity is calculated for the second detector. Gamma rays are also detected at a third detector spaced a third axial distance from the gamma ray source. The third detector is spaced farther from the source than both the first and second detectors. A third intensity is calculated for the third detector. Gamma rays are also detected at a fourth detector spaced a fourth axial distance from the gamma ray source. The fourth detector is spaced farther from the source than the first, second and third detectors. A fourth intensity is calculated for the fourth detector. An estimated formation density, cement thickness, cement density and casing thickness is then calculated based on the first, second, third and fourth intensity signals and by assuming constant values for the density and P.sub.e of the casing, negligible values for the P.sub.e of the cement and the P.sub.e of the formation and using a correction factor for the borehole fluid.
It is therefore an object of the present invention to provide an improved formation density tool for use in cased holes.
Another object of the present invention is to provide a logging tool which is capable of measuring formation density while compensating for the effects of casing and cement.
Another object of the present invention is to provide a novel four detector formation density tool.
Another object of the present invention is to provide an improved method for measuring formation density behind casing and cement.
Another object of the present invention is to provide an improved gamma ray density tool capable of measuring cement thickness, casing thickness and cement density.
Still another object of the present invention is to improve the porosity measurement of a neutron-porosity tool in a cased hole by providing accurate values for casing thickness and cement thickness.
Yet another object of the present invention is to provide an improved formation density tool that may be used in both open and cased holes and in combination with neutron-porosity tools.
Other features and advances of the present invention will appear in the following description in which one embodiment has been set forth in detail in conjunction with the accompanying drawings.